1. Field of the Invention
The present invention relates to a hydraulic circuit for an automatic transmission.
2. Description of the Prior Art
An automatic transmission is known which has a plurality of hydraulically operated friction elements and is conditioned in a desired one of a plurality of gear positions by activating selected one or more of the plurality of friction elements. For controlling the supply of servo activating fluid to and discharge thereof from the friction elements, a hydraulic circuit is provided which includes a regulator valve which generates a line pressure, a manual valve, and a plurality of shift valves. These shift valves are controlled by a plurality of shift solenoids of the on-off controlled type. The operation of the regulator valve is modulated by the operation of a line pressure solenoid of the duty controlled type such that the magnitude of the line pressure is varied with variation in duty supplied to the line pressure solenoid. For controlling a lock-up type torque converter, the degree of engagement of a lock-up clutch is controlled in a variable manner by the operation of a lock-up solenoid of the duty controlled type. This automatic transmission is described in detail in a publication "Service Manual for NISSAN FULL-RANGE ELECTRONICALLY CONTROLLED AUTOMATIC TRANSMISSION OF THE RE4OR01A TYPE (A261 C07)" published on Mar. 1987 by NISSAN MOTOR CO., LTD.
In the known automatic transmission, there is provided a pilot valve which regulates the line pressure supplied thereto to generate a constant (pilot) hydraulic pressure which is distributed to the pressure regulating portion of the above-mentioned solenoids.
The on-off solenoid receives the pilot pressure as its base pressure and a solenoid pressure which takes a high level as high as the pilot pressure or a low level which is as high as zero pressure in response to a change in On or OFF stats of the solenoid. The duty solenoid also receives the pilot pressure as its base pressure but generates a solenoid pressure which continuously varies from the low level to the high level with variation of duty which the solenoid operates to. The duty is expressed in terms of a percentage of the time length of ON state to the time length of OFF state within a predetermined period of time.
In the hydraulic circuit, it is necessary to keep the level of pilot pressure constant to keep the predetermined relation of the level of solenoid pressure generated by the duty solenoid with respect to the duty. However, there is a potential difficulty to keep the level of the pilot pressure constant if all of on-off solenoids should switch simultaneously to drain hydraulic fluid.
Accordingly, an object of the present invention is to improve a hydraulic circuit for an automatic transmission of the above-mentioned type such that the above-mentioned potential difficulty is removed.